1. Field of the Invention
The present invention relates to valves, and more particularly, but not exclusively, to valves for use in filter presses.
2. Description of the Prior Art
In a filter press, the filtrate collected at the individual filter plates may be guided out into a collecting channel through a one-way valve. A valve of this kind permits only free outlet or shut-off (open filtration). However, for reasons of hygiene or for reasons of safety (for example due to the danger of an explosion) there are many products which cannot be subjected to open filtration. In this case the filtrate is led away through a collecting channel which is located within the filter press fittings and extends through them. This "closed filtration" has the disadvantage that the condition of the filtrate or that of the wash liquid in the next stage of the process cannot be checked at the individual plates. Therefore if a fault or a blockage occurs, it is sometimes necessary to take the whole press out of operation, or to re-filter the product.
For this reason, multi-way valves have been proposed for use in open or closed filtration processes as required, according to the nature of a product. Even in the case of closed filtration processes, checks may be made or specimens may be taken at individual filter plates. If lack of tightness occurs because of damaged filter cloths, it is possible to prevent the further outlet of impure filtrate at the individual plates by shutting off the valve. The previously proposed filtrate run-off valves usually operate on the throughput member principle, in which a throughput member with different bores according to the function of the valve is arranged so as to be rotatable about its longitudinal axis in a one-part or multi-part casing. However, for effective sealing, the throughput members have to be ground in, in a conical casing. Therefore exchange of individual parts is not possible. For good sealing the throughput members must move stiffly, and they can then be actuated only with difficulty. On the other hand, ease of movement mostly leads to lack of tight sealing.
Further, it is not possible to provide sufficiently large bores in a throughput member without disadvantageous change of the diameter of the throughput member and therefore of the overall height of the whole valve. However, since generally the valve forms the narrowest part of the whole filter run-off system, relatively large bores are advisable. The problem of insufficiently large bore cross-sections arises more particularly in the case of multi-way valves, because in such valves a plurality of channels run one above the other or side by side in the throughput member. Since it is desirable to make the channels as large as possible while keeping the overall height as low as possible, the sealing surfaces between the individual channels are reduced to a minimum, and this then causes lack of tight sealing particularly readily.
The valve should also be easy to operate; the throughput member must therefore be easy to turn. In general there is not sufficient in a filter press for long levers with a horizontal direction of rotation. The space is limited by the permissable spacing between plates.
For reasons of supervision the working position in which the valve is at any time should be easily discernable from outside. In the previously proposed valves this is possible only under certain conditions, because generally only a round or predominantly oval hand wheel can be used.
For reasons of cost, filtration valves may be composed of synthetic materials, for instance polypropylene, with good resistance to the active chemicals used in the filtration. These materials have, in general, the disadvantage of high thermal expansion, due to the nature of the material. Throughput member valves consisting of this material, therefore, either fail to produce tight sealing during the changes of temperature that occur during a filtration process, or else the thermal expansion at least disadvantageously affects the mobility, that is to say the operability of the throughput members. On the other hand, lack of tight sealing gives rise to problems in operation (spraying of chemicals) and leads to losses of material.